The present invention relates to an oscillator circuit and a control method thereof, and particularly to an oscillator circuit driven at a low voltage and a control method thereof.
A high-frequency oscillator circuit is configured of, for example, a resonant circuit in which a negative resistor, an inductor and a variable capacitance type capacitor are parallel-connected to one another. A method of controlling the oscillation frequency of such an oscillator circuit is carried out by changing the capacitance of the capacitor with a voltage or changing the mutual conductance of the negative resistor with current as described in the following non-patent document 1, for example. A field effect transistor is generally used as the variable capacitance type capacitor. Described specifically, a gate electrode of the field effect transistor is connected to the negative resistor and the inductor, and a source electrode and a drain electrode thereof are short-circuited to apply 0 to a power supply voltage. The oscillation frequency of the oscillator circuit is changed to 0% to 20% by such a variable capacitance. An oscillation frequency band necessary for radio, a shift in oscillation frequency due to a change in temperature, a shift in oscillation frequency due to variations in device characteristic, etc. are all held within this frequency range.
IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 1, January 2001, pp17–22
In the description of the above patent document, the frequency band necessary for radio can be ensured even if an ambient temperature of the oscillator circuit is varied in a range of 240 K to 400 K where the power supply voltage for driving the oscillator circuit is about 2V or more. When the power supply voltage reaches about 1V, the variable width of the oscillation frequency is reduced to about 5% to about 10%. When the oscillation frequency range of the oscillator circuit becomes narrow, the frequency band necessary for radio cannot be ensured in a temperature range normally required for the oscillator circuit, e.g., a full range of 230 K to 400 K even though the frequency band necessary for radio could be ensured at a specific temperature. Its principal cause results from the fact that the mutual conductance of each transistor constituting the negative resistor is reduced with a rise in temperature, so that the frequency for starting oscillations is lowered. That is, the oscillator circuit or a voltage-controlled oscillator becomes narrow in oscillation frequency band when its drive voltage is reduced, and cannot afford to accommodate variations in oscillation frequency due to a change in temperature in terms of the frequency. Further, when the variations in production device characteristic are taken into consideration, the accommodation of the variations in device characteristic in the narrow oscillation frequency band at the time that the power supply voltage is about 1V is considered to be almost impossible.